1. Field of the Invention
The present invention relates to a photoconductor for electrophotography, and more particularly to a photoconductor for electrophotography which includes an electroconductive substrate having thereon a photosensitive layer containing a benzidine compound.
2. Description of the Prior Art
Photoconductors for electrophotography (hereafter, sometimes referred to simply as "photoconductors") each include an electroconductive substrate having thereon a photosensitive layer containing a photoconductive material. In the electrophotographic image formation system according to Carlson, a photoconductor is subjected in the dark to corona discharge to charge the photoconductor, the surface of the charged photoconductor is imagewise exposed to light using a manuscript or copy bearing, e.g., letters and/or pictures to form a latent electrostatic image, the thus formed latent electrostatic image is developed with a toner to form a visible image, the developed toner image is transferred to a support such as a paper sheet to fix the toner image on the support. After the toner image transfer, the photoconductor is subjected to the steps of removal of the electric charge and removal of the remaining toner (cleaning), and the like to be ready for reuse for a prolonged period of time.
Therefore, photoconductors are required to have not only sufficient electrophotographic characteristics such as charge generating properties, surface charge maintaining properties in the dark, and fly-off of charges upon exposure to light (light sensitivity) but also sufficient durability upon repeated use for a long time. In addition, they are required to have sufficient resistances to changes in the environmental conditions upon their use.
Heretofore, use has been widely made of photoconductors having photosensitive layers in which inorganic photoconductive materials containing selenium, zinc oxide, cadmium sulfide or the like as a major component. However, these inorganic photoconductors have not always been satisfactory in light sensitivity, resistances to environmental conditions, non-toxicity, etc.
Besides the photoconductors for electrophotography utilizing inorganic materials, those containing organic materials, have recently been studied and developed.
Organic photoconductors are generally less toxic than inorganic photoconductors. The organic photoconductors have attracted much attention by virtue of the advantageous features of the organic materials such as transparency, flexibility, lightweight, productivity, etc., as compared with the inorganic materials. For example, Japanese Patent Publication No. 10496/1975 discloses a photoconductor composed of poly-N-vinylcarbazole and 2,4,4-trinitro-o-fluorenone while Japanese Patent Publication No. 25658/1973 described a photoconductor composed of poly-N-vinylcarbazole sensitized with a pyrylium dye. However, such conventional photoconductors are not totally sufficient for their light sensitivity and durability.
In later days, so-called function-separated type laminate photoconductors in which a charge generating layer and a charge transporting layer are provided separately have been developed. For example, Japanese Patent Publication No. 42380/1980 discloses a function-separated type photoconductor which uses chlorocyan blue and a hydrazone compound. As described above, division of the photosensitive layer into a charge generating layer and a charge transporting layer, or sharing of functions by different layers, facilitated the fabrication of photoconductors with various characteristics, further development has been made with expectation to obtaining photoconductors with high light sensitivities and high durabilities.
Generally, the following properties are important for the performance of the function-separated type laminate photoconductors.
1) The charge generating layer must have a high optical absorption coefficient, a high quantum efficiency, and the charge generated must flow to the substrate efficiently and injected into the charge transporting layer efficiently.
2) The charge transporting layer must allow the charge generated in the charge generating layer to be injected therein efficiently and to transport quickly therethrough without being trapped to off-set the surface charge.
In order to meet the above requirements intensive research has been made to develop charge generating substances and charge transporting substances having improved performances as well as combinations of a charge generating layer with a charge transporting layer which can give high injection efficiencies. Various interpretations have been made on what contributes to improved injection efficiency of the combination, but no method has been established yet that can be applied generally; actually, various attempts have been made to experimentally find optimal combinations on try-and-error basis with selecting particular combinations from various charge generating substances, charge transporting substances, binders, solvents, additives and the like.
Photoconductor for electrophotography have already been known which contain N,N,N',N'-substituted benzidines as the charge transporting substance. For example, Japanese Patent Application Laid-Open No. 27033/1978 disclosed photoconductors containing benzidine compounds such as N,N'-diphenyl-N,N'-bis(2-methylphenyl)-1,1'-biphenyl-4,4'-diamine, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine, etc. With view to improving the compatibility of the aforementioned benzidine compounds with binders, Japanese Patent Application Laid-Open No. 132955/1986 proposed the use of benzidine compounds having substituents at the 3,3'-positions of the biphenyl skeleton, such as 3,3'-dimethyl-N,N,N',N'-tetraphenyl-1,1'-biphenyl-4,4'-diamine, as a charge transporting substance.
Furthermore, Japanese Patent Application Laid-Open No. 201447/1987 and Japanese Patent Application Laid-Open No. 315751/1989 proposed the use of asymmetric benzidine compounds whose substituents at the 4,4'-positions of the 1,1'-biphenyl-4,4'-diamines are different as a charge transporting substance. These asymmetric benzidine compounds were said to have superior sensitivities, less changes in the characteristics after repeated use and less occurrence of memorization phenomenon while the machine is in a stop mode over the symmetric benzidine compounds.
The aforementioned conventional proposals relate to the use of diamines of which one or both of the amino groups are diaryl-substituted. Investigation by the present inventors on photoconductors which used such diaryl-substituted diamines as a charge transporting substance revealed that although the initial characteristics of the photoconductor containing the diamines were relatively good the characteristics became gradually deteriorated while use was repeated for a long time. The deterioration was severer when the photoconductors were used at higher temperatures for a longer time.
The deterioration of the photoconductor causes failure of reproducing sufficient image density and other defects in the case of a high speed electrophotographic machine which is operated at high internal temperatures with the photoconductor being inevitably exposed to high temperature.